KrhM 



92 



The medio-cubital cross-vein. — This is a cross-vein extending 

 from tlie media to tiie cubitus, usually near the center of tnt 

 wjng. It is designated by the abbreviation m-cu. When in 

 its typical position this cross-vein extends from a point near 

 the base of vein J/3^4 to a point near the base of vein Cu^, 



The 7tiedial cross-vein, — This is a cross-vein extending 

 from vein M^ to vein M^ ; it is designated by the abbrevia- 

 tion m. The presence or absence of this cross-vein is often 

 a character of considerable taxonomic importance. 



The arculus. — In many insects there is what appears to be 

 a cross-vein extending from the radius to the cubitus near 

 the base of the wing. This has been termed the arculus by 



writers on the 

 Odonata, and the 

 use of this term 

 has been extended 



Fig. 4. — The arculus, diagrammatic. ^ ,, , 



to all orders m 

 which there is a similar arrangement of the veins in this part 

 of the wing. The arculus is designated by the abbreviation 

 ar. Usually when the arculus is present the media appears 

 to arise from it. The fact is, the arculus is compound, being 

 composed of a section of the media and a cross-vein. The 

 structure of this part can be clearly seen in the Odonata 

 (Fig. 4). In Ryphus (Fig. 2) the arculus appears as a simple 

 cross-vein extending from the radius to the cubitus, and a 

 part of the base of the media is atrophied. 



Designation of the cells of the wing. — The thin spaces 

 of the wings which are bounded by the veins are called cells. 

 In descriptions of wings it is often desirable to refer to one 

 or more of the cells. It is necessary, therefore, to have a 

 nomenclature of the cells of the wing, as well as of the wing- 

 veins. 



Having named the wing-veins, the simplest possible 

 method of designating the cells of the wing is to apply to 



